It has been widely accepted that tall buildings change wind patterns over extended periods of time. Urban pollution and heat radiation also change the environment in a city. This changes the conditions available for storm formation and development. Recent research shows cities not only trigger storms but divide the storms into smaller storms that gather and increase, downwind of urban areas. Weatherwise magazine said Purdue University researchers reviewed 10 years of data from storms surrounding Indianapolis and determined 60 percent of daytime thunderstorms changed their characteristics as they moved over a city. In addition, storms that were 46 miles away from the city did not display the same patterns as storms that formed when similar systems passed through an urban area. Fastcast: Warm and dry.